The invention relates to a xe2x80x9csubxe2x80x9d, that is a device intended to lie or move within eg. a wellbore during operations aimed at extracting fossil fuels from subterranean formations.
The invention has particular, but not exclusive, relevance to a sub that is capable of being pumped in drilling fluid eg. within a length of drillpipe inserted into a wellbore. Other subs, to which the invention also relates, are pumpable eg. during drilling operations or along a cased wellbore that omits a drillpipe. For convenience all such subs are referred to herein as xe2x80x9cdrillpipe subsxe2x80x9d, although this term should not be construed as limiting the invention solely to subs intended for use within drillpipes.
It is often necessary when pumping fluids into and out of a well via the inside of a drillpipe to keep the fluids entirely or substantially separate from each other. One reason for requiring this is to create a fluid pressure difference between the ends of a sub, so that the sub travels in a desired direction along a length of drillpipe thereby conveying a body of fluid into the drillpipe without allowing it to mingle with other fluids therein.
For this and other fluid separation purposes it is known to employ a drillpipe sub, having one or more swab cups mounted thereon, within the drillpipe.
Another purpose of such subs is to utilise fluid pressure differentials across the swab cups, to convey equipment along the drillpipe.
A swab cup is typically a flexible, hollow frustum of a cone whose narrow end is in use pierced by the cylindrical body of a sub. The narrow end of the swab cup is sealingly secured about the sub body. The base of the frustum is in the uncompressed condition of slightly larger diameter than the inner diameter of a drillpipe.
A drillpipe sub 10 having four swab cups 12 mounted thereon is shown in FIG. 1. Each of the swab cups 12 is frustoconical and includes as its narrower end a sleeve 14. Each sleeve 14 is sealingly secured encircling the exterior of a sub body 16. When the sub 10 is within a drillpipe, frustoconical annular walls 18, that are made of a flexible material, extend outwardly from the respective seals 14 so that their peripheries lie adjacent the inner surface of the drillpipe.
In the FIG. 1 arrangement the swab cups each taper towards the downhole end of the sub. When fluid pressure (e.g. of drilling mud, etc) in the drillpipe acts on the uphole side of the sub, the free peripheries of the swab cups tend to approach and seal against the drillpipe wall thereby creating a pressure differential across the swab cups. The pressure differential pumps the sub in eg. a downhole direction along the drillpipe.
In such arrangements it is not always necessary for the swab cups to engage at their peripheries the inner surface of the drillpipe. For example when using a sub to transport equipment it is often sufficient, for the purpose of creating the pressure differential, for each swab cup periphery to provide a resistance to fluid flow from one side of the swab cups to the other.
One defect of the known swab cups is that they tend to fail when the sub is subjected to relatively low axial loads, such as arise when conveying equipment within a drillpipe.
In one mode of failure, the swab cups have been found to xe2x80x9cflipxe2x80x9d or turn inside out. For example, a swab cup for a drillpipe having an internal diameter of 70 mm (2.75xe2x80x3) will turn inside out at a differential pressure of 689 kPa (100 psi), which effectively limits the axial load which may be applied to the drillpipe sub to 2668N (600 lbs). A larger swab cup for a drillpipe having a diameter of 108 mm (4.25xe2x80x3) will turn inside out at a differential pressure of 172 kPa (25 psi) which effectively limits the total force which may be applied to the cup to 1334N (300 lbs).
In another mode of failure, the base of the cone frustum defined by each swab cup rotates relative to the drillpipe, as shown in FIG. 2. When the swab cups are used to seal the drillpipe such rotation breaks contact between the inner surface of the drillpipe and the swab cups, thereby breaking the seal. When the sub supporting the swab cups is used to convey equipment such rotation increases the clearance between the swab cups and the inner surface of the drillpipe, thereby reducing or even eliminating the above-mentioned pressure differential.
According to a first aspect of the invention there is provided a drillpipe sub comprising an elongate body member defining at least one outer periphery; a first, flexible, frustoconical member including a securing portion and a base portion that is radially spaced from the securing portion, the securing portion being fixedly secured relative to the said periphery; and a second, flexible, frustoconical member defining a further base portion, the base portions of the respective frustoconical members being secured together so that the second frustoconical member interconnects the base portion of the first frustoconical member and the body member or a further member adjacent thereto; and the second frustoconical member being moveable relative to the body member at least parallel to the length thereof.
According to a second aspect of the invention there is provided a drillpipe sub comprising a first, flexible, frustoconical member including a securing portion and a base portion that is radially spaced from the securing portion, the securing portion being fixedly securable to an elongate body member; and a second, flexible, frustoconical member defining an engaging portion and a further base portion, the said engaging portion being moveably engageable with an elongate body member and the base portions of the respective frustoconical members being secured together.
During movement of the sub along a drillpipe, the second frustoconical member supports the base portion of the first frustoconical member. This prevents the outer periphery (base portion) of the first said member from turning inside out or rotating relative to a drillpipe in which it is used. This is at least partly because the moveability in use of the second frustoconical member relative to the first frustoconical member balances the fluid pressure on each side of the second frustoconical member with the fluid pressure behind the first frustoconical member.
The flexibility of the frustoconical members permits the diameter of the sub at its widest part to adjust. The sub of the invention may be designed so that its outermost periphery seals a drillpipe; or so as merely to create sufficient resistance to fluid flow to permit pumping of the sub along the drillpipe.
When the sub of the invention is configured as defined in claim 1 (ie. including an elongate sub body) defining at least one sub body outer periphery, preferably the securing portion encircles the said outer periphery and is sealingly secured thereto.
This arrangement permits the designer of a sub according to the invention to choose whether the base of the first frustoconical member is intended to seal around the interior of the drillpipe; or permit limited flow of fluid.
In another arrangement the securing portion may be arranged to permit limited flow of fluid, via the outer periphery of the elongate sub body. This provides further flow control options.
Optionally, when the second frustoconical member interconnects the base of the first frustoconical member and a further member that lies adjacent the elongate body member, the said further member is moveable relative to the elongate body member and the second frustoconical member is secured to the said further member.
In preferred embodiments a sub body is transversely divided into two parts, that are moveable one relative to the other, to define respectively the elongate body member and the further member.
Preferably the further member is engageable with the elongate body member to limit movement of the second frustoconical member relative to the elongate body member.
This arrangement is particularly convenient when the elongate body member and the further member are formed by dividing a sub body as aforesaid, since as a result the diameters of the elongate body member and the further member are the same. This facilitates abutment of the elongate and further members together; and allows the dimensions of the first and second frustoconical members to be essentially the same.
Preferably at least the first frustoconical member, and in practice each of the frustoconical members, defines a hollow interior.
When the second, frustoconical member has a hollow interior it preferably is perforated thereby permitting balancing of fluid pressure within the hollow interior thereof with fluid pressure external thereto. This arrangement is an advantageously simple one for providing balancing of pressure on either side of the second, frustoconical member.
In another embodiment one or more of the frustoconical members may have a cellular (eg. spongy) structure.
In any event, while the flexibility of the frustoconical members confers on the sub of the invention the ability to adapt to variations in drillpipe diameters and fluid pressures, it is advantageous for the material of at least one of the frustoconical members to be resiliently deformable.
At least the first, and in preferred embodiments each, frustoconical member may include resiliently deformable folding portions that permit dilation and contraction of the frustoconical members, for example in a manner similar to that of an iris diaphragm. Preferably the material of the frustoconical members has xe2x80x9cmemoryxe2x80x9d, ie. on removal of a deforming force it returns to a preformed shape.
The provision of resilient folding portions is particularly advantageous when the drillpipe sub is used in a tapered drillpipe. During movement of the sub in the tapered drillpipe, the resilient folding portions allow the frustoconical members to contract at their bases in response to a decrease in the diameter of the drillpipe while still maintaining a pressure seal.
The resilient nature of the folding portions is such that on movement of the sub from a small diameter to a large diameter drillpipe the frustoconical members dilate at their bases as the diameter of the drillpipe increases.
The resiliently deformable folding portions may be defined by a recess, formed in a wall of a said frustoconical member, that encourages adjacent sections of the said frustoconical member to overlap one another. This may be achieved by providing an angled face at the base of each recess.